This invention relates to torque transducers for measuring the magnitude of torque in shafts, in particular rotating shafts such as found in electric power steering systems in vehicle applications.
Electric power steering systems conventionally incorporate an input shaft element, connected via an intermediate shaft and Hookes joint arrangement to the steering wheel. The input shaft therefore needs to rotate through an angle typically one to two revolutions either side of the on-centre steering position. The input shaft is at least partially surrounded by the fixed housing of the steering gear. It is a requirement of the electric power steering servo system to accurately measure the continuously varying torque in this rotating shaft. Conventionally torque applied to the shaft causes it to angularly deflect, such deflection causing one part of the shaft to angularly displace with respect to another part, and this displacement is sensed to provide a measurement of this torque.
The sensing means needs to allow for rotation of the shaft within the housing, usually employing non-contact or mechanical signal transmission means. Non-contact means include optical aperture based devices and magnetic devices such as magnetostrictive or variable reluctance couplings. Mechanical means include slidably connected potentiometers and other indicating devices.
To improve the accuracy of such sensing means a torsionally compliant coupling in the form of a torsion bar is used to connect two input members at either end of the shaft. When torque is applied between the two input members the torsion bar deflects causing an increased angular displacement, which allows the use of less sensitive, or less accurate sensing means.
Generally, the use of a torsion bar requires the use of a failsafe mechanism, being a torque-limiting device to prevent failure of the torsion bar when unavoidable torque overload conditions occur.
Such torque limiting devices are well known in the art of vehicle steering, and will therefore not be described in this specification.
The prior art, which is most closely related to that of the present invention, is described in U.S. Pat. No. 5,369,583 (Hazelden) and International Patent Application No. PCT/GB95/02017 (Lucas Industries plc) which show sensors employing optical disc apertures for measuring torque.
The essence of the present invention resides in the provision of multiple electro-magnetic radiation (EMR) transmission paths incorporating grating zones composed of alternating regions of high and low transmissivity. The grating zones reside in grating elements connected by a torsionally compliant coupling, and only one grating zone resides in any one transmission path.
These transmission paths are connected to a source of electro-magnetic radiation (EMR), typically UV, visible or IR light, which is modulated as it passes through the grating zones. The modulated EMR then generates patterns on one or more arrays of detectors sensitive to the EMR. Arrays include Charge Couple Devices (CCD), Very Large Scale Integration (VLSI) vision chips, one and two-dimensional photodetector arrays and lateral effect photodiodes (commonly referred to as PSD""s or position sensitive devices). The disposition of the patterns is a function of torque applied to the shaft, and the output of the one or more arrays can be processed to produce a measure of the torque applied to the shaft. The grating zones can be arranged axially or radially about the axis of rotation of the shaft, and are of such a nature to allow a continuous output of the arrays regardless of the angular position or angular velocity of the shaft, as the limited array dimensions may not allow the complete circumference or radial face to be viewed by the arrays at any instant in time. The advantages of such a construction over that disclosed in U.S. Pat. No. 5,369,583 and Intemational Patent Application Number PCT/GB95/02017 may arise as one or more of the following:
Firstly, as only one grating zone resides in any one transmission path, it is possible to use multiple arrays, where each grating zone creates a separate pattern on one or more arrays. Thus, for example when using two grating zones and two respective arrays, twice as much detail can be included in the patterns generated compared to a single array of the same size and resolution. This makes the use of barcode patterns feasible, which, by eliminating indeterminacy caused by aliasing, reduces the mechanical complexity and assembly accuracy required.
Secondly, the use of more than one grating zone allows the zones to be placed side by side, for example in a radial or a cylindrical configuration. Thus, the two zones can be manufactured sequentially or simultaneously in a continuous manufacturing operation after the assembly of the grating elements and torsion bar. This means that much improved relative alignment of the zones can be achieved, as subsequent assembly and calibration is not required.
Thirdly, as the zones can be placed side by side, the distance between the zone and arrays can be made much smaller, minimising the inevitable loss of accuracy due to diffraction of the EMR from the boundaries of the regions of high and low transmissivity. The invention disclosed in the prior art necessarily has one zone (called slots or apertures in the prior art) located further from the array by at least the thickness of the discs in which the slots reside.
Fourthly, as the distance between the zone and arrays can be minimised, scattering effects can be reduced to a level that allows the use of non-collimated EMR sources, without focussing optical devices. This significantly reduces the complexity, assembly alignment requirements and cost of the transducer.
Finally, the location of the grating elements side by side allows the use of well known and accurate photographic or metallising techniques, for example metal on glass without excessive loss of resolution. The use of these techniques with overlapping zones (slots or apertures) as described in the prior art may result in a larger loss of resolution or other problems from internal reflection, diffraction or degradation over time as the EMR has to travel through multiple zones with attendant attenuation or diffraction for each zone.
The present invention consists in a torque transducer comprising a rotating shaft at least partially surrounded by a fixed housing, the axis of rotation of the shaft fixed with respect to the housing, the shaft comprising first and second substantially rigid torque input members which are connected by a torsionally compliant coupling, the coupling thereby enabling angular deflection of the first torque input member relative to the second torque input member as a function of the magnitude of the torque in the shaft, the transducer also comprising one or more electro-magnetic radiation (EMR) sources and one or more arrays of EMR sensitive detectors, a first grating element attached to the first torque input member and a second grating element attached to the second torque input member, the first and second grating elements each comprising a grating zone, each grating zone comprising alternating regions of high and low transmissibility to the EMR, characterised in that only one grating zone of one grating element resides in the transmission path communicating EMR from any source to any array, a pattern is produced by incident EMR on each of the one or more arrays resulting from the alternating regions of high and low transmissibility of the one or more grating zones residing in the one or more transmission paths to that array, the pattern or patterns on the one or more arrays is processed by a processor to derive the relative angular deflection of the first and second torque input members and hence provide a measure of the magnitude of the torque in the shaft.
It is preferred that at least one of the grating elements further comprises a medium substantially transparent to the EMR, the transmission path communicating EMR from any source to any array comprises a path commencing at the respective source, passing through a first boundary of the medium, optically communicating through the medium, then exiting through a second boundary of the medium and terminating at the respective array, the grating zone interfacing with either of the first or second boundaries.
Preferably, the transducer comprises two transmission paths, each path commencing at a separate source, passing through a respective grating zone, and terminating at a separate array.
Alternatively, in certain applications, the transducer comprises two sets of transmission paths, each set comprising two or more transmission paths commencing at two or more separate sources, each set of transmission paths passing through a respective grating zone, and terminating at a separate array.
Alternatively, in certain applications, the transducer comprises two transmission paths, each path commencing at a common source, passing through the respective grating zone, and terminating at a separate array.
Alternatively, in certain applications, the transducer comprises two transmission paths, each path commencing at a separate source, passing through a respective grating zone, and terminating at a common array.
Alternatively, in certain applications, the transducer comprises two transmission paths, each path commencing at a common source, passing through a respective grating zone, and terminating at a common array.
Alternatively, in certain applications, the transducer comprises two sets of transmission paths, each set comprising two or more transmission paths commencing at two or more separate sources, each set of transmission paths passing through a respective grating zone, and terminating at a common array.
Preferably the first boundary is disposed substantially radially with respect to the axis of rotation of the shaft, and the EMR emitted by the respective source is emitted in a substantially axial direction, parallel to the axis of rotation of the shaft.
Altematively the first boundary is substantially cylindrical with a central axis collinear with the axis of rotation of the shaft, and the EMR emitted by the respective source is emitted in a substantially radial direction, perpendicular to the axis of rotation of the shaft.
Preferably the second boundary is substantially cylindrical with a central axis collinear with the axis of rotation of the shaft, and the respective array is positioned adjacent to this boundary.
Alternatively the second boundary is disposed substantially radially with respect to the axis of rotation of the shaft, and the respective array is positioned adjacent to this boundary.
Preferably the pattern or patterns is also processed by the processor to derive the angular velocity of at least one of the torque input members.
Preferably the pattern or patterns is also processed by the processor to derive the relative angular position of at least one of the torque input members.
Preferably the at least one grating zone includes features or additional regions of high or low transmissibility whose resulting pattern is also processed to derive the absolute angular position of the torque input member to which the respective grating element is attached.
In one embodiment, the at least one grating zone is arranged in the form of a succession of bar codes, and the resulting pattern on the respective array is processed to derive the absolute angular position of at least one of the torque input members.
Preferably, the resulting pattern on the respective array is processed to derive the absolute angular position of both of the torque input members and the difference in absolute angular position of the first and second torque input members is processed to provide a measure of the magnitude of the torque in the shaft.
Preferably the array comprises a one dimensional or a two dimensional array, a CCD, a VLSI vision chip or a lateral effect photodiode.
Preferably the transparent medium comprises a polymer, glass or ceramic material.
Preferably the grating zones comprise a metallic coating on the first or second boundaries.
Alternatively the grating zones comprise a coating deposited by a photolithographic process on the first or second boundaries.
Alternatively the grating zones comprise physical or chemical modification of the medium.
Preferably the transducer comprises two transmission paths, both grating elements comprising a medium substantially transparent to the EMR and at least one of the transmission paths passing through the mediums of both grating elements, but only passing through one grating zone. Preferably one of the two transmission paths passes through at least four boundaries, at least two boundaries associated with the medium of the first grating element and at least two boundaries associated with the medium of the second grating element, the one grating zone interfacing with only one of these at least four boundaries.
Preferably the transducer comprises two transmission paths, each of which passes through the first or second boundary of one grating element.